Human Immunodeficiency Virus (HIV) infection causes slow destruction of the immune system, until after several years infected people succumb to other infections against which their damaged immune system cannot protect. Although new drug treatments have made startling improvements in patients, these treatments are expensive, and in the long term a vaccine is likely to be the most successful and cost-effective measure to limit the worldwide spread of HIV infection. This application proposes to compare the immune responses induced by vaccines and natural infection, using two new high-resolution assays. During the immune response against pathogens, the immune response has a wide variety of potential mechanisms to attack and kill the pathogen. For each pathogen, certain responses are effective, whereas others are ineffective because of defense strategies by the pathogen. Therefore the immune response must choose the correct mechanism for a particular infection so that the pathogen can be eradicated with minimal damage to the host. The choice between different types of immune response is regulated in a large part by T lymphocytes secreting different sets of cytokines. In this project, we will define the T cell effector responses that are induced by vaccines for HIV, and compare these with the responses associated with normal infection and progression to disease, and with the responses in patients who are exposed to HIV but apparently not infected: i.e., some of these patients may have a protective immune response. The type of immune response that is mounted against HIV will be analyzed using new assays for detecting simultaneous secretion of multiple cytokines by individual cells circulating in the blood; and for measuring the ability of single T cells to kill virus-infected cells. The potential of HIV-specific T cells to differentiate into additional effector types will also be measured, to test the hypothesis that immune responses include many uncommitted T cells that can subsequently differentiate into divergent phenotypes. Together, this information should identify the particular immune effector functions that are induced by HIV vaccines or natural HIV infection, and also define the potential for modification of immune responses by therapeutic vaccines.